1. Field of the Invention
The present invention relates generally to a drive circuit for a bar code scanner for driving an oscillatory resonant motor therein which periodically oscillates in opposite rotary directions to impart an oscillatory motion to a scanning mirror. More particularly, the subject invention pertains to a drive circuit for a resonant motor of a bar code scanner wherein the motor is driven at its own natural resonant frequency, which requires very little current to maintain the oscillatory motion.
2. Discussion of the Prior Art
Generally, the present invention relates to a scanning arrangement located within a bar code scanning device which is operative for repetitively scanning indicia having parts of different light reflectivity, for example, bar code symbols. The scanning arrangement is preferably mounted on a single printed circuit board located within a lightweight scanning device of a hand-held housing of gun-shaped configuration which may be readily held and manipulated by a user of the scanning device.
The utilization of laser scanning devices for the scanning and reading of information provided on a target, such as a package or sale item, is well known in this particular technology and has found wide acceptance in commerce. In this connection, various types of laser scanning devices incorporate scanning heads which house optical reading systems, such as bar code readers, for the reading of information or bar code symbols on targets which are scanned by a laser beam projected from the bar code reader. In general, such laser scanning devices, especially those in the type of bar code readers, are widely employed in industry, such as manufacturing, shipping, and in retail commerce and, for example, may be permanently incorporated in the structures of check-out counters of supermarkets, whereby the items of merchandise having the bar code symbols imprinted thereon or applied thereto are passed over a fixed bar code reader located beneath the counter surface so as to provide a record for the merchant of the merchandise being purchased by a consumer, and concurrently a readout (and possibly a printed record) for the consumer.
Alternatively, the bar code reader or laser scanning device may also be constituted of an optical scanner unit which is fixedly mounted on a stand extending above a support platform or countertop on which the merchandise may be arranged, or in many instances of utilization, pursuant to a preferred embodiment of the invention, may be in the form of a miniature, lightweight and gun-shaped device having a pistol grip, and which device is normally passed over the bar code symbol which is imprinted on a sale item or target at some short distance therefrom so as to enable scanning of the information provided by the bar code symbols.
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on tile surface of an article. The bar code symbol itself is a coded pattern of indicia comprising a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflecting characteristics. The readers and scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications, in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; and 4,896,026, all of which have been assigned to the same assignee as the instant application.
Among the foregoing, and in addition thereto, of particularly advantageous construction is a scan board module for laser scanners as illustrated and claimed in U.S. Pat. No. 5,015,833, which has also been assigned to the same assignee as the instant application, and the disclosure of which is incorporated herein by reference. In this connection, U.S. Pat. No. 5,015,833 discloses a scan board module contained in a generally lightweight hand-held gun-shaped housing which includes a printed circuit board on which optical and scanning components are mounted in an optically-aligned operative relationship. The printed circuit board is mounted within the housing, and the operative scanner components thereon are fastened thereto such that they are protected from damage caused by impacts or shocks sustained during rough handling or possible dropping of the scanning device. Moreover, the modular mounting of the various operative components on a single printed circuit board as disclosed and shown in this patent, as well as that elucidated particularly with regard to the embodiment of FIG. 2 of U.S. patent application Ser. No. 520,464, allows for a simple and highly efficient construction and functioning of the scanning arrangement.
As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in a hand-held, portable laser scanning head supported by a user, which is configured to allow the user to aim the head, and more particularly, the light beam or laser beam projected therefrom, at a target and a symbol which is to be read.
The light source in a laser scanner is typically a gas laser or semiconductor laser. The use of semiconductor devices, such as a laser diode, as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a lens, to form a beam spot of a certain size at the target distance. It is preferred that the beam spot size at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
Bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique "start" and "stop" character is used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
For purpose of discussion, characters recognized and defined by a symbology shall be referred to as legitimate characters, while characters not recognized and defined by that symbology are referred to as illegitimate characters. Thus, an arrangement of elements not decodable by a given symbology corresponds to an illegitimate character(s) for that symbology.
In order to increase the amount of data that can be represented or stored on a given amount of surface area, several new bar code symbologies have recently been developed. One of these new code standards, Code 49, introduces a "two-dimensional" concept by stacking rows of characters vertically instead of extending the bars horizontally. That is, there are several rows of bar and space patterns, instead of only one row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239, which is hereby incorporated by reference.
A one-dimensional single-line scan, as ordinarily provided by hand-held readers, has disadvantages in reading these two dimensional bar codes; that is, the reader must be aimed at each row individually. Likewise, the multiple-scan-line readers produce a number of scan lines at an angle to one another so these are not suitable for recognizing a Code 49 type of two-dimensional symbols.
Moreover, Wells U.S. Pat. No. 4,902,083 discloses a low vibration resonance scanning unit for miniature optical display apparatus, in which a resonance scanning unit employs a so-called tuning fork design. A scan mirror is mounted on one arm of a tuning fork, and a counter-balancing mass is mounted on the other arm of the tuning fork. Although this provides for the oscillation of the scan mirror to generate a raster display, the structure is relatively complex in nature and necessitates a plurality of components to produce the scanning operation.
In scanning systems known in the art, the light beam is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanning functions by repetitively scanning the light beam in a line or series of lines across the symbol. The scanning component may incorporate a drive or scanning motor adapted to either sweep the beam spot across the symbol and trace a scan line across and past the symbol in a high-speed repetitive mode, or scan the field of view of the scanner, or do both.
Scanning systems also normally include a sensor or photodetector which functions to detect light reflected from the symbol. The photodetector is therefore positioned in the scanner or in an optical path in which it has a field of view which extends across and slightly past the symbol. A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal, and electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal from the photodetector may typically be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
The decoding process in known scanning systems usually works in the following way. The decoder receives the pulse width modulated digital signal from the scanner, and an algorithm implemented in software attempts to decode the scan. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator of a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a completely decoded scan is achieved or no more scans are available.
Laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of bar code reader is one which incorporates detectors based upon charge coupled device (CCD) technology. In such readers, the size of the detector is larger than or substantially the same as the symbol which is to be read. The entire symbol is flooded with light from the reader, and each CCD cell is sequentially read out to determine the presence of a bar or a space. Such readers are lightweight and easy to use, but require substantially direct contact or placement of the reader on the symbol to enable the symbol to properly read. Such physical contact of the reader with the symbol is a preferred mode of operation for some applications, or as a matter of personal preference by the user.
The present invention is related technically to the following U.S. patents and patent applications, which are commonly assigned and are in the same general technical area of drive circuits and motors for bar code scanners.
U.S. patent application Ser. No. 639,822, filed Jan. 10, 1991, discloses a drive circuit for a bar code scanner which includes a voltage drive amplifier with voltage feedback for driving a stepping motor for a scanning mirror in a laser bar code reader. The disclosed voltage drive circuit provides a highly damped, low q drive in which resonance is virtually eliminated. The stepping motor includes a rotor, a stator, a center-tapped AC winding and a DC winding. The motor drive circuit comprises a triangular waveform generator for generating a periodic triangular voltage signal. A driving circuit for the AC winding receives the periodic triangular voltage signal as an input, and includes a voltage feedback circuit in which the voltage across the AC winding is compared with and caused to follow the input voltage signal driving the AC winding. The drive circuit for the AC winding includes a first noninverting operational amplifier and a second inverting operational amplifier for driving respectively first and second phase sides of the AC winding. Each of the first and second operational amplifiers receives the triangular voltage signal as a first input signal and also receives as a second input a feedback signal representing respectively the voltage across the first and second phase sides of the AC winding. The circuit also includes first and second low output impedance emitter follower drives coupled respectively to the outputs of the first and second operational amplifiers for driving respectively the first and second phase sides of the AC winding. A drive circuit for the DC winding utilizes a microstepping voltage feedback signal obtained from the AC winding for compensating for an angular offset between the AC and DC windings, and includes an operational amplifier for combining a DC voltage drive signal with the microstepping voltage feedback signal.
U.S. Pat. No. 5,003,164, for a Portable Laser Scanning System and Scanning Methods Having a Motor Amplitude Regulator Circuit, discloses a laser bar code scanner having a high speed scanning motor controlled by a motor amplitude regulator circuit which automatically controls the amplitude of oscillation of the shaft of the high speed scanning motor. Amplitude control is accomplished by using a closed loop control system that supplies the AC stator windings of the scanning motor with periodic triangular current waveforms for energizing and de-energizing the AC stator coils. The signal that closes the control loop is a feedback signal obtained from the DC stator windings of the motor which indicates the amplitude of oscillation. By controlling the amplitude of oscillation, the high speed scanning motor can maintain a consistent scanning speed and uses less power.
U.S. Pat. No. 5,019,764, for a Portable Laser Scanning System and Resonant Motor Control Circuit, discloses a laser bar code scanner having a high speed scanning motor controlled by a resonant motor circuit which automatically controls the frequency and amplitude of oscillation of the high speed scanning motor. The frequency and amplitude control is accomplished by using a single feedback signal, which is a second harmonic of the driving current, and is obtained from the windings of the Motor, which indicates resonance and also measures the amplitude of oscillation. The circuit is utilized to tune and maintain the motor in a resonant oscillating mode. Controlling both the frequency and amplitude of oscillation of the high speed scanning motor allows the motor to maintain a consistent scanning speed and to use less power.